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Only a few months ago, there was great optimism that polio was on the verge of eradication. The world saw only 223 cases in 2012, the lowest level in history, and the Global Polio Eradication Initiative, an umbrella group, unveiled a major strategy to wipe out the disease over the next five years.

Those hopes now seem clouded by a poliovirus outbreak in Somalia and Kenya. This year, there have been 81 cases in these two countries, more than the 59 cases in the three countries where polio remains endemic: Afghanistan, Pakistan and Nigeria. The first case in this outbreak, on April 30, was a 4-month-old girl who developed symptoms of acute paralysis in Dadaab, Kenya  the site of a major refugee camp, with 500,000 people, near the Somali border; within days, a case was confirmed in neighboring Somalia, according to the World Health Organization.

Polio is a highly contagious disease that affects the nervous system and can lead to paralysis, largely among children age 5 and younger. It spreads rapidly.

The WHO has warned that the "risk to neighboring countries is deemed very high, due to large-scale population movements across the Horn of Africa and persistent immunity gaps in some areas." The worry about immunity gaps is serious. Vaccination is the most critical tool in the battle against polio, and oral vaccines are much improved, but some 500,000 children in Somalia have not been vaccinated in several years, if at all. These vulnerable children are in areas largely outside the control of Somalia's weak central government, and the fear is these regions could become a cauldron for poliovirus, fueling the outbreak for a long time. Health workers simply can't reach them.

In hopes of containing the virus, road posts are being set up along major corridors to vaccinate those going and coming from the remote areas, while fresh vaccination campaigns are targeting Mogadishu, the Somali capital.

The latest outbreak underscores how armed conflicts threaten populations not only with bullets but also with disease. In Pakistan, polio has maintained a stubborn foothold in areas roiled by violence, including the assassination of polio vaccination workers by militants. Fortunately, in Somalia there does not appear to be resistance to vaccination, just an inability to extend it to those in need.

This is not the first outbreak of its kind. When vaccination was suspended in Nigeria in 2003, the virus spread across the continent, including into the Horn of Africa, and eventually into Yemen, resulting in some 700 cases. Lessons learned from that experience are being deployed in battling the current outbreak, and it is not likely to derail the global campaign for eradication. But the virus is demonstrating a dogged resilience, and it will take enormous determination and hard work to extinguish the latest scourge.